In recent years, with a remarkable improvement of a computer performance, a communication performance required for a network communication that connects among multiple computers has been also remarkably improved. In the network communication, computers are connected with each other by the aid of cables, and a request for improvement of the performance has been met by increasing the number of cables. However, under the existing circumstances, in facilities of an industrial infrastructure field such as large-scaled manufacturing line factories or plants, a total laying length of network communication cables to be used becomes a few km, and the installation, maintenance, and management of the cables have been high in the degree of difficulty and the costs.
In order to solve problems on the installation of the cables, a communication network using a wireless communication technology has been expected. However, the wireless communication is low in disturbance tolerance in a cable communication and in the stability from the viewpoint of a communication environment change. Accordingly, the probability of failure in the wireless communication becomes high as compared with the cable communication. Also, in the wireless communication, because multiple communication devices share the same space, electric waves may influence each other and interfere with each other to deteriorate the communication performance.
That is, the communication performances represented by a bit error rate (transmission error rate per one bit of data), a latency (delay time necessary for data transmission), and a throughput (rate for data transmission) in the wireless communication network may be deteriorated with a temporal or spatial change of the electric wave environment, and data transmission in the wireless communication become unstable (that is, reliability of data transmission is reduced). In the industrial infrastructure field, particularly, because the high stability and reliability of the data transmission are required, the securement of reliability becomes a barrier to wireless communication network introduction.
As described above, as means for solving the instability of the wireless communication network, there has been proposed a multihop transmission technology in which multiple relay are arranged between the wireless communication devices (for example, combination of a communication terminal which is a data source with an access point which is a data destination), and data is transferred through the relays.
In the multihop transmission technology, when data is wirelessly transferred between the communication terminal and the access point, plural kinds of relays, wireless transfer paths, and radio channels (frequencies) to be selected can be selected. For that reason, even if a wireless communication environment of one relay, one wireless transfer path, or one radio channel is unstable, and the wireless communication performance is low, another relay, another wireless transfer path, or another radio channel can selected to stabilize data transmission between the communication terminal and the access point.
A general purpose of the multihop transfer technology is to optimize selection of the relay and the wireless transfer path so as to conduct data transmission between the terminal and the access point with high reliability and stability and efficiently.
Also, as a method for stabilizing a one-to-one wireless communication between the respective wireless communication devices using the multihop transfer technology, there have been proposed redundancy methods such as retransmission control for transmitting the same data plural times until the data arrives at a destination, and encoding control for adding an error correcting code to the data. A redundancy device increases the amount of transmission of given data to improve the reliability and stability of the wireless communication.
On the other hand, as a method for stabilizing the wireless communication, there has been proposed a method in which data is compressed, and a time required for the wireless communication is shortened, to thereby improve the stability. An influence of disturbance on the wireless communication may be reduced more as one wireless communication time is shorter. Further, that the time required for the wireless communication is shortened enables a power consumption to be reduced because an operating time of the wireless communication device can be shortened.
Hereinafter, a description will be given of an example of the multihop transfer technology, and the method of stabilizing the wireless communication including data compression.
For example, a method of compressing data according to a communication state has been disclosed (for example, refer to JP-A-Hei7 (1995)-203053). In the method of JP-A-Hei7 (1995)-203053, data to be transmitted is classified into video data and other data (control data, emergency message data, audio data, and general data). Also, a communication quality is determined according to a level of the received data or received electric wave. When it is determined that the communication quality is low, the video data is compressed and the other data is made redundant on the basis of the classified results whereby the method of JP-A-Hei7 (1995)-203053 ensures the communication reliability of the other data even if the video data is sacrificed.